Thin Film Deposition Controllers which are based on Quartz Crystal Microbalance (QCM) have been in use for a very long time in the thin film coating industry such as described, for example, in U.S. Pat. No. 5,112,642 to Wajid and U.S. Pat. No. 5,869,763 to Vig et al., among others. In a typical arrangement, a monitor quartz crystal is placed in proximity to a substrate in a thin film deposition apparatus wherein the crystal and substrate are each coated at the same time. The material that is deposited on the crystal is usually proportional to the material which is deposited on the substrate. As a result of material deposition on the quartz crystal, its resonance frequency shifts downwards in a monotonic manner. Therefore, the knowledge of the frequency shift of the crystal along with the density of the material allows an estimation of the thickness of material that is deposited on the crystal and the substrate.
Previously, the subject matter of mass determination by a piezoelectric quartz crystal microbalance has been handled, for example by Lu and Czanderna in their treatise, Applications of Piezoelectric Quartz Crystal Microbalances. Earliest QCM instruments utilized the Sauerbrey relation in order to calculate deposited film's thickness from the resonant frequency shift of the quartz crystal. Sauerbrey's formula was accurate, however, only for a very limited frequency shift. In the 1970s, Lu and Lewis published an analysis that accounted for the elastic properties of the deposited film. The Lu-Lewis equation, trademarked as Zmatch® vastly improved the accuracy of QCM instruments and extended their useful range. As a result, nearly all QCM instruments intended for thin film deposition process control currently use the Zmatch equation for converting the frequency shift of a quartz crystal into the thickness of the deposited film.
However, the Zmatch equation is strictly valid with regard to the deposition of only one material on a quartz crystal. Deposition of two or more dissimilar materials in succession, compromises the accuracy of this relation. The extent of error depends on the extent of mismatch of the acoustic properties of deposited materials and the thickness of the layers that are deposited. Therefore and if a process requires different materials to be deposited on a substrate, a dedicated quartz crystal must be used exclusively for each material; particularly, if accuracy is paramount.
Over the years, the market for QCM-based thin film controllers have evolved. In recent times, its predominant use is found in the optical coating industry. An optical coating is usually a stack of many thin layers of dielectric materials, mostly oxides and fluorides. These materials are dissimilar in terms of their optical as well as their acoustic properties. Circumstances rarely permit users to dedicate one quartz crystal for each deposited material. However, compromising the accuracy of thickness/measurement is something that can not be afforded. As a result, optical coating houses use QCM for the deposition rate control and determine deposition process using an optical end-point detector, such as a reflectometer or ellipsometer.
In the early 1990s, the inventors of the present application created a process referred to as “Auto-Z”, which partly overcame the above-noted limitation. However, Auto-Z is only an approximation that is based upon simultaneous evolution of two resonant frequencies of a quartz crystal. To that end, Auto-Z is useful in the deposition of multiple layers of materials with known acoustic properties, or materials of unknown acoustic properties or alloys of varying stoichiometry. However, Auto-Z is only an incremental improvement over the Zmatch equation and is not a substitute for an exact solution of the problem of multi-layer thin film deposition.
To the best of our knowledge, only thin film controllers such as those that are commercially sold by ULVAC Corporation presently make claims of multi layer control. A review of the commercially available ULVAC Model CRTM-9000 Controller indicates use of a linear extrapolation scheme. When a layer of different material is deposited, the slope of extrapolation is recalculated. All these calculations are done prior to actual deposition of the current layer. As described in the User's Manual for this controller, it may take up to tens of seconds to finish these background calculations. During deposition, the last calculated slope is multiplied by the frequency shift in order to estimate the thickness of the current layer. Thus, it is safe to conclude that these controllers do not use an analytically correct solution for multi-layer deposition.
There is also an ancillary problem associated with the deposition of thin film on a quartz crystal. The above-noted Zmatch equation requires precise knowledge regarding the specific acoustic impedance or its inverse ratio to that of AT-cut quartz (z-ratio) of the material to be deposited. The accuracy of the estimation of thickness/rate therefore directly depends on the accuracy of this physical property. The extent of error depends on the extent of mismatch of acoustic properties of deposited materials and the total thickness of the layers.
Another significant and growing use of QCM is in the emerging organic light emitting diode (OLED) manufacturing industry. OLED processes usually require use of fairly exotic organic materials. Some of these materials in fact are so new that their physical properties, such as, elastic modulus, shear wave velocity, specific acoustic impedance or even density are not definitively known.
A conventional Zmatch technique can be used to back calculate z-ratio or specific acoustic impedance of an unknown material, provided a thick layer (i.e., one micron or more) is deposited on the quartz crystal. This is necessary to minimize the impact of acoustic impedance mismatch at the electrode-film boundary. OLED materials, on the other hand, are light weight and highly damping. Thus, the deposited layers are often thin and the associated mass load is less than that of the electrode itself. Thus, conventional methods will be highly error prone in this case.